1. Field of the Invention
This invention relates in general to the construction of devices for evaporating substances and, in particular, to a new and useful device for electron-beam evaporation.
2. Description of the Prior Art
The present invention relates to a device for electronbeam evaporation, comprising a holding structure for the substance to be evaporated, a hot cathode, a focusing and an accelerating electrode for the electrons emitted by the cathode, and means for generating a magnetic guide field deviating the electrons to the substance to be evaporated, with an ion-collector being provided for collecting the ions formed in the vapor beam during the evaporation. Such devices are used for the vacuum depositing of thin layers. In most cases, it is necessary to control the evaporation rate during the deposit, that is, the quantity of evaporating substance which is evaporated from the evaporation source during a given period of time, for example, a second, divided by this period of time. The evaporation rate is decisive for the evaporation velocity or the so-called coating rate, i.e., the quantity of layer substance deposited on the substrate during a period of time divided by this period of time. Frequently, a physical quantity depending on the coating rates is measured, for example, the increase in thickness of the layer or the frequency variations of a quartz resonator exposed to the vapor beam, and used as a measure for the coating rate and also for the evaporation rate.
The accurate and rapid measuring of the evaporating rate is important primarily in cases where this measurement has to serve as a basis for controlling the evaporation capacity of the vapor source in conformity with the requirements of the evaporation process, for example, for keeping this capacity constant or for varying the same in time in accordance with a predetermined program. The last-named control may be necessary, for example, for depositing a layer having a refraction gradient.
A device for the continuous measuring of the evaporation rate is known which includes an ion-detector projecting into the zone of the vapor beam and supplied with a constant direct current for furnishing a test-current depending on the vapor beam density and thereby affecting the evaporation rate. With the ion-detector located in the vapor beam, this device has the disadvantage of intercepting a part of the vapor and throwing a vapor-beam shadow on the object to be coated by evaporation. However, to eliminate this shadow formation, the ion-collecting electrode may be disposed off the vapor-sourceobject path, vapor-source/object example, closely adjacent the object to be coated.
Nevertheless, experience has shown that surprisingly, such ion-detectors located in the space above the evaporation source do not offer the desired accuracy and reproducibility of measurement to be expected. That is, after a prolonged use of such a detector, the observation is made that at the same evaporation rate and under otherwise apparently entirely equal conditions, nature of the evaporated substance, residual gas pressure, voltage applied to the detector, measured values are obtained differing from the initial values.